In the past, a semiconductor device having an IGBT, which functions as a switching element, and a FWD integrated into one chip has been employed in an inverter circuit or the like that performs direct-to-alternating current conversion. The direct-to-alternating current conversion is performed by turning ON and OFF the IGBT. In addition, when the IGBT is turned OFF, a current flowing into a load (for example, a motor) is circulated through the FWD.
In a FWD employed in such a semiconductor device, a characteristic of a forward current If with respect to a forward voltage Vf (hereinafter referred to as the Vf-If characteristic) at the time the IGBT remains OFF is linear. However, the Vf-If characteristic at the time the IGBT is turned ON has a nonlinear domain due to an effect of a snapback voltage. FIG. 6 is a diagram showing the Vf-If characteristic of the FWD. As shown in the drawing, when the IGBT is turned ON, compared with when the IGBT is OFF, the forward voltage Vf fluctuates. In addition, there is a domain in which the snapback voltage is generated. Therefore, linearity is not attained.
Specifically, assuming that a semiconductor device has an IGBT and a FWD that are formed into one chip, an equivalent circuit attained when the IGBT is turned ON can be illustrated as shown in FIG. 7. That is, a channel resistor Ra of an IGBT 100 and an internal resistor Rb from a channel to a pn junction (an under part of a p-type deep well layer 13) of a FWD 200, which is constructed with the p-type deep well layer 13 and a n−-type drift layer 1, are connected in parallel with the FWD 200. Further, an internal resistor Rc of the n−-type drift layer 1 in the under part of the p-type deep well layer 13 and an internal resistor Rd of a field stop (hereinafter referred to as the FS) layer 1a are connected in series with the channel resistor Ra and internal resistor Rb. In this type of circuit, when the IGBT 100 is turned on, a current flows to the channel side of the IGBT 100 rather than to the diode side due to the small internal resistor Rb. Therefore, conductivity modulation does not occur so that the forward voltage Vf can increase.
Therefore, in the semiconductor device including the IGBT 100 and the FWD 200, gate control is performed in such a manner that a gate voltage is not applied to the IGBT 100 during operation of the FWD 200. In order to determine the operation of the FWD 200, diode sensing is performed. For accurate sensing, a linear Vf-If characteristic is required. Therefore, there is need to perform the diode sensing by avoiding the nonlinear domain susceptible to the snapback voltage. That is, the sensing can be applied only in a domain where the forward voltage Vf is large.
The patent document 1 proposed a method of reducing an increase in Vf by increasing the width of a p-type base region 4 that does not have a gate structure.